This story is co-published with Grist, and is part of Record High, a Grist series examining extreme Heat and its impact on how—and where—we live.Cities are hot. When you cover the ground with asphalt and concrete, jam millions of cars together on congested streets, and erect thousands of buildings that leak their own heat, you create what experts call an “urban heat island.” Daytime temperatures in these places can be as much as 7 degrees Fahrenheit higher than surrounding rural areas, and things don’t get much cooler at night.As climate change fuels a succession of historic heat waves, the urban heat island effect in many American cities is pushing the limits of human survivability. That’s the case in desert cities like Phoenix, where temperatures crested 110 degrees F for 30 straight days this summer, and also in cooler climes like Chicago, which has seen a series of scalding triple-digit days over the past few months.Dealing with this type of heat requires more than isolated interventions — reflective roofs here or mist machines there. Rather, the crisis of the past summer has shown that most American urban centers will need to consider a revamp from the ground up.Drawing on feedback from climate experts, architects, and urban planners, as well as successful technologies pioneered by warm-weather communities around the world, Grist set out to design a city built from scratch to handle extreme heat, all while reducing cooling-related carbon emissions. The buildings and streets in this cool metropolis incorporate basic design principles such as shade and foliage, but they also include bespoke architectural solutions like wind-trapping towers and special absorbent polymers. The finished product shows how much work is needed to adapt to the extremes of climate change, but it also shows how much more humane and people-oriented our cities can be.With their tall towers of steel, abundant concrete, and frequent lack of green space, dense downtown areas are known for scorching temperatures that last day and night. But there are many design elements that can help cool these cityscapes, from shaded sidewalks and bus stops to reflective glass. Even the positioning of tall buildings, such as those on a street like Manhattan’s Fifth Avenue, can create what is known as an “urban canyon,” blocking the sun from reaching the street during the morning and afternoon. This keeps the pavement cool for most of the day, and reduces the risk of heat stroke and overexposure.“You can’t generalize and say that density is bad,” said Sara Meerow, an associate professor of urban planning at Arizona State University who studies heat risks. “If you plan your density well, you can build in ways that are not going to increase heat risks.”But most American space is not high-rise. More than half of the U.S. population lives in what they identify as more of a suburban area, peppered with single-family homes and low-rise office buildings in layouts that often rely on car travel. Many of the solutions that apply to dense cities, such as shade and trees, can be applied here, but these communities also have different challenges — and solutions. For one, more than half of all the energy used in single-family homes comes from heating and air conditioning. Overall, home cooling accounts for 6 percent of total electricity consumption in the U.S. Building smarter, more heat-resilient homes, particularly with sustainable or natural materials, can lower temperatures and energy bills, and reduce the associated carbon emissions that come with AC use.“That is the future,” said Turner. “We are a ways off from it, but we need to be coming up with ways to make scalable building homes with [natural] materials. As a general rule of thumb, the more unnatural the material is, the more it becomes like hot pavement.”Researchers are also increasingly experimenting with so-called phase-change materials, substances that melt at specific temperatures, thereby absorbing heat from the surrounding area and cooling it down. Paraffinic waxes and different types of salt hydrates are some common types of phase-change materials. When injected into walls, floors, and roofs, they’ve been found to lower temperatures by up to 7 degrees Fahrenheit. One study in Casablanca, Morocco, found that when the roofs of homes contained a copolymer made of ethylene and paraffin, temperatures indoors declined by 2.7 degrees. While there are a few companies offering construction materials infused with phase-change materials on the market, it hasn’t yet found widespread commercial success, and the Department of Energy has several studies underway to assess the effectiveness of the technology.Outside of skyscraper-filled downtowns and leafy suburban neighborhoods, most American cities also contain expanses of factories, shopping, warehouses, and industrial sites — areas that can’t be ignored in the planning for heat-proof cityscapes. Big-box stores and strip malls may blast AC on the inside, but they tend to be barren of trees, congested with heat-emitting cars, and surrounded by asphalt parking lots. The same is true for manufacturing areas, where power plants and factories also leak heat into surrounding streets. Solving this thermal buildup isn’t as simple as installing a mister or increasing airflow — urban designers need to plan from the ground up with reflective materials and ample shade to reduce the urban heat island effect and make these commercial spaces safe for daily use as temperatures climb.